Method for synthesis of metal nanoparticles

ABSTRACT

Metal nanoparticles containing two or more metals are formed by heating or refluxing a mixture of two or more metal salts, such as a metal acetates, and a passivating solvent, such as a glycol ether, at a temperature above the melting point of the metal salts for an effective amount of time.

RELATED APPLICATIONS

This application is Divisional application of U.S. patent applicationSer. No. 10/304,316, filed on Nov. 26, 2002, and is related toco-pending U.S. patent application Ser. No. 10/304,342 and U.S. patentapplication Ser. No. 10/304,317, all of which are incorporated byreference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a method for the synthesis of metalnanoparticles.

BACKGROUND

Metal nanoparticles are an increasingly important industrial material.Due in part to their high surface area and high reactivity, metalnanoparticles may be used in a variety of applications, such as reactioncatalysis (including serving as a reaction substrate), improving thebehavior and properties of materials, and drug delivery. Particularapplications for nanoparticles include serving as a catalyst for thesynthesis of carbon nanotubes, serving as a catalyst for hydrogen gassynthesis, and production of metal hydrides.

Many techniques are currently used for the production of metalnanoparticles. Current techniques include plasma or laser-driven gasphase reactions, evaporation-condensation mechanisms, and various wetchemical techniques. This plurality of techniques is due in part to thefact that no current technique provides a reliable, simple, and low-costmethod for production of nanoparticles of a controlled size. Somecurrent techniques may produce particles of a desirable size, but withpoor crystallinity or an unpredictable distribution of phases within thenanoparticles. Other techniques suffer from an inability to control thedistribution of sizes around a desired nanoparticle size. Still othernanoparticle synthesis techniques require specialized equipment, longprocessing times, or expensive specialty chemicals.

One potentially attractive wet chemical technique for synthesis of metalnanoparticles is thermal decomposition, as these reactions may becarried using relatively simple equipment. However, currently knownmethods of metal nanoparticle formation using thermal decompositionrequire addition of a separate surfactant, thus increasing thecomplexity and cost of the method.

What is needed is a simple, reliable, and low cost thermal decompositionmethod for producing crystalline metal nanoparticles without use of aseparate surfactant that allows for control of the average particle sizewhile minimizing the amount of variance in the particle sizes.

SUMMARY

The present invention provides a method for the synthesis of metalnanoparticles containing two or more types of metal via a thermaldecomposition reaction. In an embodiment of the invention, two or moremetal acetates or other suitable metal salts are placed in separatereaction vessels. A suitable passivating solvent, such as a glycolether, is also added to each reaction vessel. The contents of thereaction vessels are mixed for a period of time to form a substantiallyhomogenous mixture within each vessel. After forming a substantiallyhomogenous mixture in each reaction vessel, the contents of the reactionvessels are combined into a single reaction vessel. The contents of thisreaction vessel, containing at least two types of metal salt, are mixedto again form a substantially homogenous mixture. The contents of thereaction vessel are then refluxed at a temperature above the meltingpoints of the metal salts to form metal nanoparticles. The desiredcomposition of the synthesized metal nanoparticles is achieved bycontrolling the concentrations of the metal salts in the passivatingsolvent. The desired particle size of the synthesized metalnanoparticles is achieved by controlling the concentration of the metalsalts in the passivating solvent and by varying the amount of refluxtime.

In another embodiment of the invention, two or more metal acetates orother suitable metal salts are placed in a reaction vessel with apassivating solvent such as a glycol ether. The contents of the reactionvessel are mixed for a period of time to form a substantially homogenousmixture. The contents of the reaction vessel are then refluxed at atemperature above the melting points of the metal salts to form metalnanoparticles. The desired composition of the synthesized metalnanoparticles is achieved by controlling the concentrations of the metalsalts in the passivating solvent. The desired particle size of thesynthesized metal nanoparticles is achieved by controlling theconcentration of the metal salts in the passivating solvent and byvarying the amount of reflux time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one example of an apparatus for use in carrying out thepresent invention.

FIG. 2 shows another example of an apparatus for use in carrying out thepresent invention.

FIG. 3 a depicts a flow chart for a method of producing metalnanoparticles according to an embodiment of the present invention.

FIG. 3 b depicts a flow chart for a method of producing metalnanoparticles according to another embodiment of the present invention.

FIGS. 4 a-4 e show histograms of metal nanoparticle sizes for metalnanoparticles produced via an embodiment of the present invention.

FIGS. 5 a-5 e show histograms of metal nanoparticle sizes for metalnanoparticles produced via another embodiment of the present invention.

FIG. 6 shows X-ray diffraction spectra of bimetallic nanoparticlesproduced according to various embodiments of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 and 2 depict possible apparatuses that may be used for carryingout the present invention. While FIGS. 1 and 2 depict possible equipmentselections, those skilled in the art will recognize that any suitablemixing apparatus and reflux apparatus may be used. Although nospecialized equipment is required to carry out the present invention,the components used should be suitable for use with the variousembodiments of this invention. Thus, the equipment should be safe foruse with organic solvents and should be safe for use at the refluxtemperature of the thermal decomposition reaction.

In FIG. 1, a mixing apparatus is generally shown at 100. Reaction vessel130 may be any suitable vessel for holding the metal salt andpassivating solvent mixture during the mixing and reflux steps of thepresent invention. In an embodiment, reaction vessel 130 may be a 500 mlglass or Pyrex™ Erlenmeyer flask. Other styles of reaction vessel, suchas round-bottom flasks, may also be used as long as the reaction vesselis compatible for use with the mixing and reflux apparatuses. In theembodiment shown in FIG. 1, reaction vessel 130 is attached to sonicator150. Sonicator 150 may be used to mix the contents of reaction vessel130. A suitable sonicator is the FS60 available from Fisher Scientificof Pittsburgh, Pa. In other embodiments, the contents of reaction vessel130 may be mixed by other methods, such as by using a standardlaboratory stirrer or mixer. Other methods of mixing the solution willbe apparent to those skilled in the art. Reaction vessel 130 may also beheated during mixing by a heat source 170. In FIG. 1, heat source 170 isshown as a hot plate, but other suitable means of heating may be used,such as a heating mantle or a Bunsen burner.

FIG. 2 depicts a reflux apparatus 200. In this apparatus, reactionvessel 130 is connected to a condenser 210. Condenser 210 is composed ofa tube 220 that is surrounded by a condenser jacket 230. During a refluxoperation, water or another coolant is circulated through condenserjacket 230 while heat is applied to reaction vessel 130. The coolant maybe circulated by connecting the inlet of the condenser jacket to a waterfaucet, by circulating a coolant through a closed loop via a pump, or byany other suitable means. During reflux, evaporated passivating solventrising from reaction vessel 130 will be cooled as it passes through tube220. This will cause the passivating solvent to condense and fall backinto reaction vessel 130. Note that the method of connecting condenser210 with reaction vessel 130 should form a seal with the top of reactionvessel so that gases rising from the reaction vessel must pass throughtube 220. This can be accomplished, for example, by connecting condenser210 to the reaction vessel 130 via a stopper 205. The end of tube 220 ispassed through a hole in stopper 205. As in FIG. 1, heat source 170 maybe a hot plate, heating mantle, Bunsen burner, or any other suitableheating apparatus as will be apparent to those skilled in the art.

In other embodiments of the invention, both mixing and reflux may beaccomplished using a single apparatus. For example, stopper 205 may havea second opening to allow passage of the shaft of the stirring rod froma laboratory mixer or stirrer. In these embodiments, once the metal saltand passivating solvent have been added to reaction vessel 130, thereaction vessel may be connected to the dual mixing and refluxingapparatus. Still other embodiments of how to mix and reflux the contentsof a reaction vessel will be apparent to those skilled in the art.

FIG. 3 a provides a flow diagram of the steps for an embodiment of thepresent invention. FIG. 3 a begins with preparing 310 a mixture byadding a passivating solvent and a metal salt to a reaction vessel. Notethat depending on the choice of metal salt and passivating solvent, thismixture could be in the form of a solution, suspension, or dispersion.In an embodiment, the passivating solvent is an ether. In another,embodiment, the passivating solvent is a glycol ether. In still anotherembodiment, the passivating solvent is 2-(2-butoxyethoxy)ethanol,H(OCH₂CH₂)₂—O—(CH₂)₃CH₃, which will be referred to below using thecommon name dietheylene glycol mono-n-butyl ether. In yet anotherembodiment, the passivating solvent is a combination of two or moresuitable solvents, such as a combination of two different glycol ethers.Additional substances that may serve as the passivating solvent will bediscussed below.

In an embodiment, the metal salt will be a metal acetate. Suitable metalacetates include transition metal acetates, such as iron acetate,Fe(OOCCH₃)₂, nickel acetate, Ni(OOCCH₃)₂, or palladium acetate,Pd(OOCCH₃)₂. Other metal acetates that may be used include molybdenum.In still other embodiments, the metal salt may be a metal salt selectedso that the melting point of the metal salt is lower than the boilingpoint of the passivating solvent.

As will be discussed below, the relative amounts of metal salt andpassivating solvent are factors in controlling the size of nanoparticlesproduced. A wide range of molar ratios, here referring to total moles ofmetal salt per mole of passivating solvent, may be used for forming themetal nanoparticles. Typical molar ratios of metal salt to passivatingsolvent include ratios as low as about 0.0222 (1:45), or as high asabout 2.0 (2:1). In an embodiment involving iron acetate and diethyleneglycol mono-n-butyl ether, typical reactant amounts for iron acetaterange from about 5.75×10⁻⁵ to about 1.73×10⁻³ moles (10-300 mg). Typicalamounts of diethylene glycol mono-n-butyl ether range from about 3×10⁻⁴to about 3×10⁻³ moles (50-500 ml).

In another embodiment, more than one metal salt may be added to thereaction vessel in order to form metal nanoparticles composed of two ormore metals. In such an embodiment, the relative amounts of each metalsalt used will be a factor in controlling the composition of theresulting metal nanoparticles. In an embodiment involving iron acetateand nickel acetate as the metal salts, the molar ratio of iron acetateto nickel acetate is 1:2. In other embodiments, the molar ratio of afirst metal salt relative to a second metal salt may be between about1:1 and about 10:1. Those skilled in the art will recognize that othercombinations of metal salts and other molar ratios of a first metal saltrelative to a second metal salt may be used in order to synthesize metalnanoparticles with various compositions.

In still another embodiment, preparing a mixture 310 may involve aseries of steps, such as those shown in the flow diagram in FIG. 3 b.FIG. 3 b begins with initially preparing 311 two or more mixtures ofmetal salt and passivating solvent in separate reaction vessels. In anembodiment, each mixture is formed by adding one metal salt to apassivating solvent. In preferred embodiments, the same passivatingsolvent is used to form each of the metal salt and passivating solventmixtures. After preparing the passivating solvent and metal saltmixtures in the separate reaction vessels, the contents of each of thereaction vessels are mixed during initial mixing 315. During initialmixing 315, the contents of the reaction vessels are mixed to createsubstantially homogeneous mixtures. The homogenous mixtures may be inthe forms of mixtures, solutions, suspensions, or dispersions. In anembodiment, the contents of the reaction vessels are sonicated for 2hours. In another embodiment, the contents of the reaction vessel may bemixed using a standard laboratory stirrer or mixer. Other methods forcreating the homogeneous mixture or dispersion will be apparent to thoseskilled in the art. The contents of the reaction vessel may be heatedduring initial mixing 315 in order to reduce the required mixing time orto improve homogenization of the mixture. In an embodiment, the contentsof the reaction vessels are sonicated at a temperature of 80° C. Afterfirst mixing 315, the homogenous mixtures are combined 320 into a singlereaction vessel to create a mixture containing all of the metal saltsand passivating solvents.

Returning now to FIG. 3 a, after preparing 310 a mixture containing allmetal salts and passivating solvents in a single reaction vessel, thecontents of the reaction vessel are mixed during mixing 330. Duringmixing 330, the contents of the reaction vessel are mixed to create asubstantially homogeneous mixture of metal salt in the passivatingsolvent. The homogenous mixture may be in the form of a mixture,solution, suspension, or dispersion. In an embodiment, the contents ofthe reaction vessel are mixed by sonication. In another embodiment, thecontents of the reaction vessel may be mixed using a standard laboratorystirrer or mixer. The contents of the reaction vessel may also be heatedduring mixing 330 in order to reduce the required sonication or mixingtime. In an embodiment, the contents of the reaction vessel aresonicated at 80° C. for two hours and then both sonicated and mixed witha conventional laboratory stirrer at 80° C. for 30 minutes. In anotherembodiment, the contents of the reaction vessel are sonicated at roomtemperature for between 0.5 and 2.5 hours. Other methods for creatingthe homogeneous mixture will be apparent to those skilled in the art.

After forming a homogenous mixture, metal nanoparticles are formedduring the thermal decomposition 350. The thermal decomposition reactionis started by heating the contents of the reaction vessel to atemperature above the melting point of at least one metal salt in thereaction vessel. Any suitable heat source may be used including standardlaboratory heaters, such as a heating mantle, a hot plate, or a Bunsenburner. Other methods of increasing the temperature of the contents ofthe reaction vessel to above the melting point of the metal salt will beapparent to those skilled in the art. The length of the thermaldecomposition 350 will be dictated by the desired size of the metalnanoparticles, as will be discussed below. Typical reaction times mayrange from about 20 minutes to about 2400 minutes, depending on thedesired nanoparticle size. The thermal decomposition reaction is stoppedat the desired time by reducing the temperature of the contents of thereaction vessel to a temperature below the melting point of the metalsalt. In an embodiment, the reaction is stopped by simply removing orturning off the heat source and allowing the reaction vessel to cool. Inanother embodiment, the reaction may be quenched by placing the reactionvessel in a bath. Note that in this latter embodiment, the temperatureof the quench bath may be above room temperature in order to preventdamage to the reaction vessel.

In a preferred embodiment of the invention, the contents of the reactionvessel are refluxed during the heating step. In this embodiment, astandard reflux apparatus may be used, such as the one depicted in FIG.2. During the thermal decomposition 350, water (or another coolant) ispassed through condensing jacket 230. Vapors rising from the passivatingsolvent are cooled as they pass through tube 220, leading tocondensation of the passivating solvent vapors. The condensedpassivating solvent then falls back into the reaction vessel. Thisrecondensation prevents any significant loss of volume of thepassivating solvent during the thermal decomposition reaction. Thus, therelative ratio of metal to passivating solvent stays substantiallyconstant throughout the reaction. Those skilled in the art willrecognize that while refluxing is a preferred method for carrying outthe thermal decomposition reaction, it is not necessary for nanoparticleformation. As long as the temperature of the homogeneous mixture israised to above the melting point of the metal salt, the desired thermaldecomposition reaction will take place and lead to formation of metalnanoparticles.

After forming the metal nanoparticles in the thermal decomposition 350,the metal nanoparticles are removed from the passivating solvent for useduring nanoparticle extraction 370. The nanoparticles may be removedfrom the passivating solvent by a variety of methods. Those skilled inthe art will recognize that the best method for extracting thenanoparticles may depend on the desired application. In an embodiment, aportion of the metal nanoparticle/passivating solvent mixture is mixedwith ethanol. A suitable volume ratio for this mixture is 1 partpassivating solvent to 5 parts ethanol. This mixture is then heated to atemperature below the melting point of the metal salt to evaporate thesolvent and leave behind the metal nanoparticles. In another embodiment,the passivating solvent is directly evaporated away by heating the metalnanoparticle/passivating solvent mixture to a temperature where thepassivating solvent has a significant vapor pressure. In still anotherembodiment, the nanoparticles remain in a thin film of the passivatingsolvent that is left behind after evaporation.

In another embodiment, particles of aluminum oxide (Al₂O₃) or silica(SiO₂) may be introduced into the reaction vessel after the thermaldecomposition reaction. A suitable Al₂O₃ powder with 1-2 μm particlesize and having a surface area of 300-500 m²/g is available from AlfaAesar of Ward Hill, Mass. During nanoparticle extraction 370, Al₂O₃powder is added to the metal nanoparticle/passivating solvent solution.In an embodiment, enough powdered oxide is added to achieve a desiredweight ratio between the powdered oxide and the initial amount of metalused to form the metal nanoparticles. In an embodiment, this weightratio is between roughly 10:1 and roughly 15:1. After adding the Al₂O₃powder, the mixture of nanoparticles, powdered Al₂O₃, and passivatingsolvent is sonicated and mixed again to create a homogenous dispersion.The mixture is then heated to evaporate off the passivating solvent. Inan embodiment involving dietheylene glycol mono-n-butyl ether as thepassivating solvent, the mixture is heated to 231° C., the boiling pointof the passivating solvent. Evaporating the passivating solvent leavesbehind the metal nanoparticles deposited in the pores of the powderedAl₂O₃. This mixture of Al₂O₃ and metal nanoparticles is then ground upto create a fine powder. This method of removing the metal nanoparticlesfrom solution may be used when the metal nanoparticles will besubsequently used for growth of carbon nanotubes.

Note that some nanoparticle extraction techniques 370 will modify thecharacteristics of the metal nanoparticles themselves. Metalnanoparticles are highly reactive, in part due to their high surfacearea to volume ratio. When certain types of metal nanoparticles areexposed to an environment containing oxygen, especially at temperaturesabove room temperature, the metal nanoparticles will have a tendency tooxidize. For example, iron nanoparticles extracted from a passivatingsolvent by heating the passivating solvent to 230° C. in the presence ofoxygen will be at least partially converted to iron oxide nanoparticles.Thus, even though the present invention relates to the synthesis ofmetal nanoparticles, it is understood that the metal nanoparticles maysubsequently become partially oxidized after the completion of thethermal decomposition reaction.

The size and distribution of metal nanoparticles produced by the presentinvention may be verified by any suitable method. One method ofverification is transmission electron microscopy (TEM). A suitable modelis the Phillips CM300 FEG TEM that is commercially available from FEICompany of Hillsboro, Oreg. In order to take TEM micrographs of themetal nanoparticles, 1 or more drops of the metalnanoparticle/passivating solvent solution are placed on a carbonmembrane grid or other grid suitable for obtaining TEM micrographs. TheTEM apparatus is then used to obtain micrographs of the nanoparticlesthat can be used to determine the distribution of nanoparticle sizescreated.

FIGS. 4 a-4 e and 5 a-5 e depict histograms of particle sizedistributions for iron nanoparticles created under several conditions.The particle size distributions represent iron nanoparticles made bymixing iron acetate and diethylene glycol mono-n-butyl ether in areaction vessel to form a homogeneous mixture. The contents of thereaction vessel were then refluxed at the boiling point of diethyleneglycol mono-n-butyl ether (231° C.) for the time period specified ineach figure. The figures also note the concentration of the metalacetate in the passivating solvent. The concentrations are specified asratios of milligrams of iron acetate per milliliter of passivatingsolvent, but note that these ratios are coincidentally similar to themolar ratios, due to the similar molecular weights of iron acetate anddiethylene glycol mono-n-butyl ether (173.84 g/mol versus 162.23 g/mol)and the fact that the density of diethylene glycol mono-n-butyl ether isclose to 1.

Two factors used to control the size distribution of the nanoparticleswere the concentration of metal in the passivating solvent and thelength of time the reaction was allowed to proceed at the thermaldecomposition temperature. FIGS. 4 a-4 e depict histograms from a seriesof reactions where the ratio of milligrams of iron acetate tomilliliters of diethylene glycol mono-n-butyl ether was held constant at1:1.5 while varying the length of the reflux at the reactiontemperature. For comparison purposes, the histograms have beennormalized so that the area under the histogram bars in each figureequals 100. FIG. 4 a depicts results from the shortest reaction time of20 minutes at the boiling point of diethylene glycol mono-n-butyl ether(231° C.). As shown in FIG. 4 a, 20 minutes of thermal decompositionreaction time leads to a narrow distribution of particle sizes centeredon 5 nm. FIGS. 4 b-4 e depict similar histograms for increasing amountsof reaction time. As seen in the figures, increasing the reaction timeleads to an increase in the average particle size. Additionally, FIGS. 4d and 4 e indicate that at the longest reflux times (300 minutes and1200 minutes), the width of the particle size distribution alsoincreases.

FIGS. 5 a-5 e provide additional results from thermal decompositionreactions with varying concentrations at a constant reaction time of1200 minutes, or 20 hours. Note that even the lowest ratio of ironacetate to passivating solvent results in an average particle size of 10nm. These results indicate that both low concentrations and shortreaction times are required to achieve the smallest particle sizes.

When more than one type of metal salt is used in the synthesis of themetal nanoparticles, the composition of the resulting metalnanoparticles may be determined by using X-ray diffraction (XRD). Asuitable XRD tool is a Bruker D-8 X-ray diffractometer available fromBruker-AXS GMBH of Karlsruhe, Germany. A sample of metal nanoparticlescan be prepared for XRD analysis by placing a drop of metalnanoparticle/passivating solvent mixture on a measurement substrate,such as an SiO₂ substrate. The passivating solvent is then evaporatedaway by heating the substrate to 250° C., leaving behind the metalnanoparticles.

FIG. 6 shows a comparison of XRD spectra for metal nanoparticles formedby thermal decomposition of a mixture of iron acetate and nickel acetatein diethylene glycol mono-n-butyl ether. The relative molar ratio ofiron to nickel within the mixture was about 2:1, while the relativemolar ratio of metal acetate to passivating solvent was 1:1.5. Thediffering spectra are the result of differences in the preparation 310and mixing 330 of the initial metal salt/passivating solvent mixtures.For spectrum a), the metal salt/passivating solvent mixture was preparedby adding iron acetate, nickel acetate, and diethylene glycolmono-n-butyl ether to a single reaction vessel. The contents of thereaction vessel were then sonicated for two hours at room temperature.Metal nanoparticles were then formed by refluxing the metalsalt/passivating solvent mixture at 231° C. for 3 hours. For spectrumb), the metal nanoparticles were synthesized by first preparing separatemixtures of iron acetate in diethylene glycol mono-n-butyl ether andnickel acetate in diethylene glycol mono-n-butyl ether. These separatemixtures were sonicated at 80° C. for two hours. After this, the ironacetate/diethylene glycol mono-n-butyl ether mixture and the nickelacetate/diethylene glycol mono-n-butyl ether were combined in a singlereaction vessel. This combined mixture was both mixed and sonicated at80° C. for 30 minutes. Metal nanoparticles were then formed by refluxingthe combined mixture at 231° C. for 3 hours.

A comparison of spectra a) and b) in FIG. 6 shows that the differingprepartion 310 and mixing 330 procedures influenced the composition ofthe resulting metal nanoparticles. Note that due to the preparationtechnique used for obtaining the XRD spectra, the metal nanoparticlescontaining iron were at least partially oxidized. In FIG. 6, spectrum a)shows a series of peaks that are believed to represent crystallographicfaces of NiFe₂O₄ particles. These peaks are identified with arrows.These same peaks are also visible in spectrum b), where some of thepeaks are identified by the crystallographic face assigned to the peak.However, spectrum a) also shows several additional peaks identified withasterisks which have no counterpart in spectrum b). These peaks arebelieved to represent crystallographic faces of Ni particles.

Without being bound by any particular theory, it is believed that thedifferences between spectra a) and b) are the result of improvedhomogenization of the metal salt/passivating solvent mixture. The metalnanoparticles synthesized for spectrum b) were initially prepared inseparate vessels and sonicated (and mixed) at a higher temperature thanthe metal nanoparticles synthesized for spectrum a). Additionally, thetotal sonication and mixing time for the metal nanoparticles synthesizedfor spectrum b) was greater than that for spectrum a). It is believedthat the additional mixing and sonication prevented the formation of thesegregated Ni metal nanoparticles observed in spectrum a). Note,however, that the metal salt/passivating solvent mixture used to preparethe metal nanoparticles in spectrum a) was still sufficientlyhomogenized to allow metal nanoparticle formation during the thermaldecomposition reaction.

The embodiments discussed thus far have shown production of metalnanoparticles based on thermal decomposition of metal acetates indiethelyne glycol mono-n-butyl ether. However, the method may be moregenerally used with other combinations of metal salts and passivatingsolvents. Without being bound by any particular theory, it is believedthat the present invention involves a thermal decomposition reaction ofone or more metal salts in a passivating solvent. Because no additionalsurfactant is added to the reaction, the passivating solvent is believedto serve as a passivating agent that controls the growth of the metalnanoparticles. When a metal acetate is used as an initial metal salt,the acetate groups may also assist with passivation. However, it isbelieved that metal salts other than metal acetates may be selected solong as the melting point of the metal salt is lower than the boilingpoint of the passivating solvent. Suitable metal salts may include metalcarboxylate salts.

As for the passivating solvent, without being bound by any particulartheory, it is believed that the passivating solvent acts to preventagglomeration of larger metal clusters during the thermal decompositionreaction. It is believed that as the metal salt decomposes, the smallestsizes of nanoclusters begin to nucleate. These small nanoclusters arehighly reactive and would quickly aggregate into larger clusters ofvarious sizes in the presence of a non-passivating solvent. It isbelieved that the passivating solvent binds to the surface of thenanoclusters and retards the growth and aggregation of the nanoclusters.In order to achieve this passivating effect, it is believed that thepassivating solvent must be of a sufficient size and the solventmolecules must be composed of a minimum ratio of oxygen to carbon atoms.Thus, other organic molecules may be suitable as a passivating solventas long as they meet several conditions. First, the passivating solventmust be a liquid with a sufficiently low viscosity in the vicinity ofthe melting point of the metal salt used in the thermal decompositionreaction. In addition to having a boiling point above the melting pointof the metal salt, the passivating solvent must have low enoughviscosity at a temperature below the melting point so that it isfeasible to create the homogenous dispersion described above. Second,the individual passivating solvent molecules must be of a sufficientsize. For straight chain molecules, such as diethylene glycolmono-n-butyl ether, the individual molecules should have a molecularweight of at least 120 g/mol. This minimum may vary for branchedmolecules depending on the nature and type of the branching. Forexample, a t-butyl type carbon group would be unlikely to assist inpassivation of the surface of a metal nanoparticle, so moleculesinvolving this type of molecular group would likely require a higherminimum molecular weight. Third, the individual passivating solventmolecules must have a sufficient ratio of oxygen to carbon within themolecule. In order to achieve passivation, ether linkages andcarboxylate groups are more likely to exhibit passivating behavior thanalcohol groups, so solvents such as the glycol ether described abovewould be preferred over molecules having a similar molecular weight thatonly contain alcohol functional groups. In the glycol ether describedabove, the ratio of oxygen to carbon atoms is 3:8. Other organicmolecules with oxygen to carbon ratios near 1:3 may also be suitable.Thus, passivating solvents with oxygen to carbon ratios from 1:2 to 1:4should be suitable. Additionally, it was noted above that the acetategroups provided when a metal acetate is selected as the metal salt mayparticipate in passivating behavior. If a metal salt is selected that isnot a metal acetate, passivating solvents with higher oxygen to carbonratios, as well as greater numbers of ether and carboxylate functionalgroups, would be preferred.

While the invention has been particularly shown and described withreference to a preferred embodiment and various alternate embodiments,it will be understood by persons skilled in the relevant art thatvarious changes in form and details can be made therein withoutdeparting from the spirit and scope of the invention.

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 41. Metal nanoparticlesproduced by the process comprising the steps of: providing a mixtureconsisting essentially of one or more metal salts and a passivatingsolvent; mixing the mixture of said one or more metal salts and saidpassivating solvent; and heating the mixture of said one or more metalsalts and said passivating solvent to a temperature above the meltingpoint of at least one of said one or more metal salts and maintainingthe temperature above the melting point of at least one of said one ormore metal salts to form metal nanoparticles.
 42. The metalnanoparticles produced by the process of claim 41, wherein at least onemetal salt is a transition metal acetate.
 43. The metal nanoparticlesproduced by the process of claim 41, wherein at least one metal salt isa metal carboxylate.
 44. The metal nanoparticles produced by the processof claim 41, wherein at least one metal salt is a substance selectedfrom the group consisting of iron acetate, palladium acetate, nickelacetate, and molybdenum acetate.
 45. The metal nanoparticles produced bythe process of claim 41, wherein the passivating solvent is a glycolether.
 46. The metal nanoparticles produced by the process of claim 41,wherein the passivating solvent is 2-(2-butoxyethoxy)ethanol.
 47. Themetal nanoparticles produced by the process of claim 41, wherein thepassivating solvent comprises a mixture of glycol ethers.
 48. The metalnanoparticles produced by the process of claim 41, wherein heating themixture of said one or more metal salts and said passivating solventcomprises the step of refluxing the mixture of said one more metal saltsand said passivating solvent.
 49. The metal nanoparticles produced bythe process of claim 48, wherein the mixture of said one or more metalsalts and said passivating solvent is refluxed at the boiling point ofsaid passivating solvent.
 50. The metal nanoparticles produced by theprocess of claim 41, wherein the molar ratio of metal salt andpassivating solvent in the mixture of said one or more metal salts andsaid passivating solvent is between about 2:1 and about 1:45.
 51. Themetal nanoparticles produced by the process of claim 41, wherein themolar ratio of a first metal salt and a second metal salt in the mixtureof said one or more metal salts and said passivating solvent is betweenabout 1:1 and about 1:10.
 52. The metal nanoparticles produced by theprocess of claim 41, wherein mixing the mixture of said one or moremetal salts and said passivating solvent comprises the step of mixingthe mixture of said one or more metal salts and said passivating solventto form a homogenous mixture.
 53. The metal nanoparticles produced bythe process of claim 41, wherein the mixture of said one or more metalsalts and said passivating solvent is mixed using a sonicator.
 54. Themetal nanoparticles produced by the process of claim 41, wherein thetemperature of the mixture of said one or more metal salts and saidpassivating solvent is maintained at a temperature above the meltingpoint of at least one of said one or more metal salts for a time betweenabout 20 minutes and about 2400 minutes.
 55. The metal nanoparticlesproduced by the process of claim 41, wherein providing a mixtureconsisting essentially of a plurality of metal salts and a passivatingsolvent comprises the steps of: providing a first mixture consistingessentially of a first metal salt and a first passivating solvent;providing a second mixture consisting essentially of a second metal saltand a second passivating solvent; mixing said first mixture to form afirst substantially homogenous mixture; mixing said second mixture toform a second substantially homogenous mixture; and combining said firstsubstantially homogenous mixture and said second substantiallyhomogenous mixture in a single reaction vessel.
 56. The metalnanoparticles produced by the process of claim 55, wherein at least oneof the first passivating solvent and the second passivating solvent is2-(2-butoxyethoxy)ethanol.
 57. The metal nanoparticles produced by theprocess of claim 41, wherein mixing the mixture of said one or moremetal salts and said passivating solvent comprises the steps ofsonicating the mixture of said one or more metal salts and saidpassivating solvent for 2 hours at 80° C. followed by sonicating andstirring the mixture of said one or more metal salts and saidpassivating solvent for 30 minutes at 80° C.
 58. Metal nanoparticlesproduced by the process comprising the steps of: providing a mixture ofa plurality of metal acetates and a passivating solvent; mixing themixture of said plurality of metal acetates and said passivatingsolvent; heating the mixture of said plurality of metal acetates andsaid passivating solvent to a temperature above the melting points ofsaid plurality of metal acetates and maintaining the temperature abovethe melting points of said plurality of metal acetates to form metalnanoparticles; and extracting the metal nanoparticles from thepassivating solvent.